Add Builder::emit_struct_field_null to output null

pbjson-build/src/generator/message.rs

@@ -42,6 +42,7 @@ pub fn generate_message<W: Write>(
     btree_map_paths: &[String],
     emit_fields: bool,
     preserve_proto_field_names: bool,
+    emit_struct_fields_null: bool,
 ) -> Result<()> {
     let rust_type = resolver.rust_type(&message.path);
 
@@ -54,6 +55,7 @@ pub fn generate_message<W: Write>(
         writer,
         emit_fields,
         preserve_proto_field_names,
+        emit_struct_fields_null,
     )?;
     write_serialize_end(0, writer)?;
 
@@ -116,6 +118,7 @@ fn write_message_serialize<W: Write>(
     writer: &mut W,
     emit_fields: bool,
     preserve_proto_field_names: bool,
+    emit_struct_fields_null: bool,
 ) -> Result<()> {
     write_struct_serialize_start(indent, message, writer, emit_fields)?;
 
@@ -127,6 +130,7 @@ fn write_message_serialize<W: Write>(
             writer,
             emit_fields,
             preserve_proto_field_names,
+            emit_struct_fields_null,
         )?;
     }
 
@@ -404,6 +408,7 @@ fn write_serialize_field<W: Write>(
     writer: &mut W,
     emit_fields: bool,
     preserve_proto_field_names: bool,
+    emit_struct_fields_null: bool,
 ) -> Result<()> {
     let as_ref = format!("&self.{}", field.rust_field_name());
     let variable = Variable {
@@ -443,6 +448,22 @@ fn write_serialize_field<W: Write>(
                 writer,
                 preserve_proto_field_names,
             )?;
+            if emit_struct_fields_null {
+                writeln!(writer, "{}}} else {{", Indent(indent))?;
+
+                let json_name = field.json_name();
+                let field_name = if preserve_proto_field_names {
+                    field.name.as_str()
+                } else {
+                    json_name.as_str()
+                };
+                writeln!(
+                    writer,
+                    "{}struct_ser.serialize_field(\"{}\", &None::<Option<()>>)?;",
+                    Indent(indent + 1),
+                    field_name,
+                )?;
+            }
             writeln!(writer, "{}}}", Indent(indent))?;
         }
         FieldModifier::Repeated | FieldModifier::UseDefault => {

pbjson-build/src/lib.rs

@@ -108,6 +108,7 @@ pub struct Builder {
     emit_fields: bool,
     use_integers_for_enums: bool,
     preserve_proto_field_names: bool,
+    emit_struct_field_null: bool,
 }
 
 impl Builder {
@@ -200,6 +201,12 @@ impl Builder {
         self
     }
 
+    /// Output optional struct fields with None value as "null", instead of omitted.
+    pub fn emit_struct_field_null(&mut self) -> &mut Self {
+        self.emit_struct_field_null = true;
+        self
+    }
+
     /// Generates code for all registered types where `prefixes` contains a prefix of
     /// the fully-qualified path of the type
     pub fn build<S: AsRef<str>>(&mut self, prefixes: &[S]) -> Result<()> {
@@ -289,6 +296,7 @@ impl Builder {
                             &self.btree_map_paths,
                             self.emit_fields,
                             self.preserve_proto_field_names,
+                            self.emit_struct_field_null,
                         )?
                     }
                 }